Surfacing is the deposition of filler metal on the surface of a base metal. Its purpose is to provide the properties or dimensions necessary to meet a given service requirement. There are several types of surfacing. They may be categorized as cladding, hardfacing, buildup, and buttering. The desired properties, in the same order are corrosion resistance, wear resistance, dimensional control, and metallurgical needs.
Cladding is a relatively thick layer of filler metal applied to a carbon or low alloy steel base metal for the purpose of providing a corrosion-resistance surface when that surface is to be exposed to a corrosive environment. Hardfacing is a form of surfacing that is applied for the purpose of reducing wear or abrasion, impact, erosion, galling, or cavitation. Buildup, as it is normally used, connotes the addition of weld metal to a base metal surface, the edge of a joint, or a previously deposited weld metal for the restoration of the component to the required dimensions. Buttering also connotes the addition of one or more layers of weld metal to the face of the joint or surface to be welded. It differs from buildup in that its use is for metallurgical reasons, not dimensional control. Buttering is used especially when joining dissimilar metals and also for joining carbon steel to a low alloy steel when stress relief of the completed weld is to be avoided. The buttered member can be heat-treated after buttering, or it can be left in the as-welded condition.
Surfacing is often used to improve, update, repair or rework a part so it will have properties equal to or in excess of the original part. In many cases, special problems and considerations are necessary for surfacing. For example, the chemical composition and mechanical properties of the surfacing material may be quite different from those of the workpiece on which it is deposited and a relatively large area of the workpiece is covered in surfacing. If the surfacing uses small amounts of material, there may be a large gradient in alloy and carbon content and mechanical properties across the fusion line between the base metal and the surfacing metal.
Furthermore, dilution, as expressed as a percentage which is equal to the amount of workpiece metal melted divided by the sum of the filler material and workpiece material, the quotient of which is multiplied by 100, may also be an important consideration in surfacing.
In addition to these special requirements, there are many other problems which must be dealt with in a surfacing process. The process should be accurately controllable to be predictable so that known, uniform deposition layers can be produced. Furthermore, the process should not be dangerous to the operator or to others in the vicinity nor should it endanger the workpiece being surfaced. This is an especially important requirement for delicate or small workpieces such as small thin wires, ball bearings or the like, and is also important if the surfacing process is to be amenable to correcting defects in other surfacing processes. In addition to the above, to be entirely effective, the surfacing process should be expeditious and economical and capable of being carried out inhouse. This last requirement is best satisfied by a process which requires a minimum number of steps yet is adaptable to use in a variety of situations to provide a surface which is proper for the workpiece and use expected for that workpiece. Furthermore, from a metallurgical point of view, the composition and properties of surfacing are strongly influenced by the dilution obtained.
In the past, most surfacing has been done using one of the consumable electrode arc welding processes such as disclosed in U.S. Pat. No. 3,184,578. Because of the importance of dilution, it is necessary that the effect of each consumable electrode arc welding variable be known. Many of the welding variables that affect dilution and, therefore, require close control in surfacing are amperage, polarity, electrode size, electrode extension, pitch, electrode oscillation, travel speed, position of workpiece, contamination, and the like. Control of all of these variables may be difficult and, if not properly carried out, undesirable results may be produced.
As welding requires some means for heating or pressing materials to produce a coalescence of those materials, such process has many problems. For example, the heating required may establish severe thermal stresses which may warp or damage the workpiece, and damage the surface produced. The success of a surfacing application sometimes depends upon the magnitude of the internal stresses and whether or not the external stresses are shear, tensile, or compressive. Residual stresses from the welding operation may add to or oppose any stresses encountered in service and, thus accentuate or increase any tendency of the surfacing to crack. Such damage may require further processes or operations to correct. This is especially troublesome if the workpiece is small or delicate. Furthermore, the heating processes may be dangerous to the operator or others in the vicinity and may have to be carried out in a carefully controlled environmental under carefully controlled conditions.
Due to the various problems associated with welding, a particular type of welding process is not versatile and should only be applied in specific situations with specific workpieces. For this reason, there is a wide variety of welding processes, such as arc welding, solid state welding, resistance welding, soldering, and brazing available. Even within these overall catagories, there are numerous types of processes available, such as atomic hydrogen welding, bare metal arc welding, carbon arc welding, cold welding, diffusion welding, flash welding, plasma arc welding, arc brazing, percussion welding, electron beam welding, laser beam welding, and the like. While it may appear to be desirable to have such a wide variety of processes available, such variety is indicative of the non-adaptability of the welding process itself. Such non-adaptability may result in economic and manufacturing problems for a surfacing process.
In addition to the above problems with welding processes in general, an arc welding process in which an arc is used as the means for heating the materials, has additional problems of being essentially an unknown quantity. The characteristics, mechanisms, laws and phenomena associated with arc welding are not fully understood. Therefore, it is difficult to predict ahead of time, what the exact result of an arc welding process will be, and the unknown characteristics of arc welding make it difficult to apply it in a wide variety of situations as it must be carefully controlled in each application. Thus, it is possible to apply too much power and thus damage the workpiece, or too little power and perform an inadequate surfacing process, among other problems. An inadequate surfacing process may be a problem in many situations, but is is also unacceptable in other situations, such as would occur in a food can equipment surfacing situation. Furthermore, the high temperature of a welding arc creates special problems as discussed above with certain workpieces. An arc is also dangerous to people in the area, and may be a source of pollution and be wasteful of energy.
Therefore, welding process for surfacing workpieces have many drawbacks.
To avoid the major problems associated with welding in particular, and with the thermal stresses established in the workpiece in particular, but still obtain the advantages of welding, the welding process has been modified in some surfacing applications. Thus, processes such as spark electrodeposition have been developed. This process of metal transfer by short duration electric discharge applied repetitively at a high rate has been known for a long time. In a spark electrodeposition, a spark discharge is employed which is effected when a electrode is brought into and/or out of contact with a metallic surface to be treated, with a brief electrical impulse applied between them which is of an intensity sufficient to effect localized heating of the relatively small discharge-impinging area, and, by sweeping such contact discharge over a selected surface region of the workpiece, a metallurgical modification or hardening of this elected surface area is obtained. Both electrode and workpiece are conductive and form the terminal poles of a direct current power source. Linear vibration of the electrode is commonly used to provide relative motion between the two poles. It is typically achieved by mounting the electrode to the armature of a solenoid. Typical operation of such a solenoid is 120 cycles per second. Alternate motions of the electrode have been attempted, including rotation of the electrode about its axis and combinations of rotation and linear vibration to provide intermittent contact with the surface, thereby causing repetitive electrical discharges.
The individual discharges through the electrode must be of short duration (less than 100 microseconds) and the energy level at 0.01 to 4 joules for material transfer to satisfactorily occur. A condition known as arcing occurs when the electrical discharge is of low intensity and long duration. Duration of the spark during arcing can be of a magnitude of one hundred times longer than is desirable for spark deposition purposes. It therefore becomes very desirable to minimize the duration of the spark while maintaining the selected energy level for proper deposition.
The actual mechanism involved in material transfer by electrodeposition is subject to speculation. However, it is usually assumed that due to the spark itself or to the I.sup.2 R heating associated therewith, a gas bubble forms about the spark discharge and persists for a time longer than the area adjacent to the spark itself. Metal in either the electrode and/or the workpiece melted due to the high temperature is transferred from the electrode to the workpiece surface via the expanding gas bubble. This theory has the electrode tip cooling before the workpiece and being drawn away to break the weld thus formed and leave material from the electrode tip deposited on the workpiece. Generally, maximum material transfer is made by moving the electrode over the workpiece and by making the electrode the anode and the workpiece surface the cathode of the discharge circuit. Another reason for providing relative motion between the electrode and workpiece is the need to continually fracture the adhesive junctions forming as the electrical discharge occur and the molten metal deposits and solidifies.
However, while avoiding the severe thermal stresses often associated with bulk heating of the workpiece in the above-mentioned welding processes, the spark treatment processes still require heating, albeit local heating, of the workpiece. This situation may be acceptable in many applications, but in small, delicate parts, even localized heating of the workpiece to melting temperature may be unacceptable. Furthermore, in some situations, the sparks can be dangerous to personnel in the area, this may be especially true in the situation where the spark carries material from the electrode to the workpiece. Such situations can come very close to being a pulsed arc technique and thus be subject to the above-discussed welding associated problems. As mentioned above, a full understanding of the spark electrodeposition principles and theory is not available. Therefore, like welding, spark electrodeposition is also subject to problems associated with this lack of understanding.
Examples of the spark electrodeposition process are disclosed by Antonov in U.S. Pat. No. 3,832,514, Balskowski in U.S. Pat. Nos. 3,360,630, 3,277,267, and Inoue in U.S. Pat. No. 3,098,150. The Blaskowski and Inoue patents use electrode-surface contact to establish an intense localized current heating of the surface for producing the coalescence of materials mentioned above. The Antonov process maintains an electrode spaced a definite distance from the work surface and maintains a spark arc to deposit material on that surface. This arc will be established by a potential high enough so material can be transferred by the arc. In all of these processes, significant heating of a small workpiece may occur. In Blaskowski and Inoue, the current heating, even though local, will occur and could endanger small elements; and in Antonov, the material-transporting spark arcs will have enough energy to heat the workpiece in a manner similar to welding arcs. In addition, the Inoue technique may require a rather substantial impact to be delivered to the workpiece and the Blaskowski process may only treat a small upper layer of the workpiece surface. The Antonov process is not as severe as arc welding but due to the use of a material-carrying arc, it is similar enough to such technique to be subject to the above-discussed problems and drawbacks associated with welding, especially if used on small workpieces. Certain variables in the Antonov technique can be changed in much the same manner as the aforediscussed welding variables of amperage, welding speed, and the like. However, the Antonov variables are changed within the context of the arc welding type operation being performed and even though electric potential for example is altered, it cannot fall below a value sufficient to establish a material-carrying spark arc. However, even such a low energy material-carrying arc will cause heating of the workpiece which may be significant enough to endanger a small item.
Other examples of spark electrodeposition are disclosed by E. H. Thorton and R. G. Davies in an article in "Metals Technology" copyrighted 1979 by the Metals Society, 1 Carlton House, Terrance, London SWIYSDB, and U.S. Pat. Nos. 3,097,291, 3,098,150, 3,316,381, 3,617,680, 3,741,426, 3,969,601, 3,524,596, 3,614,373, 3,415,971, 3,415,970, 4,098,447, 4,205,211 and 4,292,494 as well as British Pat. No. 756727, Japanese patent specifications 32-9998 issued Nov. 29, 1957, 32-599 issued Jan. 29, 1959, 32-2446 issued Apr. 19, 1959, 32-2900 issued May 16, 1959 and 32-6848 issued Aug. 28, 1959.
In all of the above mentioned processes, the associated heating of the workpiece may be significant enough to endanger items like switch relays, razor blade edges or the like. Therefore, known spark electrodeposition processes have many drawbacks which may make them unacceptable for use with delicate workpieces.
Because of these and other drawbacks, other surfacing processes such as thermal spraying, adhesive bonding, vacuum deposition, sputtering, and the like, have been developed in an effort to produce adequate surfacing without the problems associated with welding or welding-type techniques. However, these processes have drawbacks due to the quantity and quality of the finished product and also often produce irregular and unpredictable results as well as porous or weakly bonded coatings, which require further processes to correct. These processes may also be expensive and of limited applicability, and difficult to repetitively apply in-house.
Recently, ion beams have been used to surface treat metals in a pollution-free manner which is suitable for use on small, delicate items. Ion beams can implant microscopic diffusions of other materials into a thin surface layer thereby creating a new compound. Therefore, claddings and ultra-thin coatings are possible. However, surface treating using ion beams may require a plasma of ions which is accelerated by an electric potential of up to 100 kilovolts. Ion beams can be used for plating, or other surface treatments, such as roughening etching or the like. However, such surface treatment may be extremely expensive and of very limited application.
Rocklin in U.S. Pat. No. 3,763,343 discloses a metal treating tool which uses a spark to locally heat and melt the metal surface of the workpiece. The electrode is not consumed and heating is extremely localized and is quickly air quenched so that the heating itself changes the workpiece surface to surface treat that workpiece. This device is inexpensive and is capable of in-house use on small items; however, the electrode is not consumed and the surface treatment is expressly limited to changing surface characteristics only, thereby limiting the use of this tool to roughening or smoothing surfaces only. Therefore, this tool cannot perform surfacing such as cladding, hardfacing, buttering and buildup, or the like which require deposition of material on the surface.
Therefore, there is need for a surfacing process which is different from welding and hence is not subject to the drawbacks of welding or spark treating techniques which require heating of the workpiece, yet which is inexpensive and produces a well-bonded surface coating to small, delicate items.